Diffraction

 

Introduction

When light passes through a narrow slit it spreads out, this is called diffraction. The light does not spread out uniformly but forms bright and dark areas as shown below.

 

The width of the central maximum depends on the size of the slit, small slit gives wide maximum. With the apparatus set up as shown below the relationship between the angle θ and the slit size is:

bsinθ = λ

If the angles are small then sinθ=y/D
so the equation becomes: by/D=λ

 

Research question

What is the relationship between the size of the slit and the width of the central maximum?

Independent variable: Slit width
Dependent variable:Central maximum width
Controlled variables: wavelength, slit-screen distance.

 

Method

Using the Vernier calliper as a slit form a diffraction pattern on the wall. By measuring the width of the central maximum and the size of the slit use a graphical method to show that by/D=λ and find λ.

 

Using a vernier calliper

 

If you haven't used the vernier calliper before you will need a quick tutorial on how to use it.

To read the vernier scale you first read the position of the 0 on the slider. In the example the slider points at something between 2 and 3 mm. To find the next significant figure we look to see where the sliding scale coincides with the fixed scale. This is at 5 on the sliding scale. The reading is therefore 2.5mm.

For more practice at using the vernier callipers you can visit this site:

Vernier applet

 

Warning

The laser is a powerful source of parallel light if you look directly at the beam it will be focused as a very small spot on your retina. This can burn a hole leaving a permanent blind spot. You may have tried this with a laser pointer but the lab laser is more powerful so don't try it with this one.

 

 

 

 

 

 

 

 

 

 

 

DCP Diffraction